While Mexico as a whole has made many improvements in access for the blind and those in wheelchairs, Chiapas [1] is still a very hard place to navigate if you have a disability. Smaller towns are the most problematic, as their sidewalks can be narrow and even some main streets are not paved. Definitely ask for help—for what Mexico lacks in infrastructure, its people make up for in graciousness.
Links:
[1] http://www.moon.com/destinations/chiapas